Hepatocellular carcinoma (HCC) is the sixth most common cancer worldwide and the third leading cause of cancer-related deaths. At present, there are limited options in treating HCC patients. There are urgent needs to develop effective therapies for HCC. Both farnesoid x receptor (FXR) and estrogen receptor ? (ER?) signaling are linked to HCC. A large body of evidence support a view that FXR and ER? signaling provide protection against HCC development. FXR knockout (FXR-KO) mice spontaneously developed HCC as they aged while ER?-KO mice exhibited increased susceptibility to chemical-induced HCC. Consistent with their protective roles in HCC, FXR and ER? signaling were dysregulated or defective in large percentages (60-80%) of HCC patients with decreased or total lack of FXR or ER? expression with concurrent switches to its variants or different isoforms. In our preliminary studies with FXR-KO, ER?-KO and double FXR and ER? knockout (FXR/ER?-DKO) mice, we discovered that FXR and ER? signaling crosstalked each other through coordinately regulating a novel oncogene ubiquitin specific peptidase 2 (USP2) in the development of HCC. The overall objective of this proposal is to understand the interplay of FXR and ER? signaling in HCC development and the underlying mechanisms. The central hypothesis, built on our extensive preliminary results, is that FXR and ER? signaling have both tumor-protective and promoting activities dependent on the status of the other signaling through their regulation of USP2 in a reciprocal antagonistic manner. Specific Aim 1 is to delineate the interplay of FXR and ER? signaling in HCC development. Specific Aim 2 is to determine the oncogenic roles of USP2 as a downstream target of FXR and ER? signaling in the development of HCC. Specific Aim 3 is to investigate the mechanistic insights into the intriguing crosstalk between FXR and ER? signaling in regulating USP2. The study represents a pioneering effort to delineate the complex and interactive nature of FXR and ER? signaling in HCC development. The experiments proposed are built on our extensive, robust and novel preliminary findings as well as our long-standing experience in studying FXR and ER? signaling and their interaction in liver diseases including cholestasis and HCC. With newly generated FXR/ER?-DKO mice, our laboratory is thus uniquely poised to investigate the interplay of FXR and ER? signaling in HCC development. Implementation of these innovative concepts and findings is expected to greatly enable the advancement of developing novel therapies for HCC.